This invention relates to a device for processing digital image data, in particular a device for encoding and decoding image data.
In recent years, devices have been developed for compressing image data based on standards such as JPEG, MPEG-1, MPEG-2, H261, H263, and reproducing the image data. Image encoding devices based on these standards offer a high data compression ratio while maintaining high image quality, and are therefore expected to become more widespread.
As this compressed data is digital data, there is almost no deterioration of quality even if the data is repeatedly duplicated, or copied. It is therefore possible, instead of expending the effort required to produce original data, to illegally duplicate data which is already commercially available and release any part of it onto the market as if it was original data. To counter this trend, producers of the original data are for example scrambling their data in order to prevent unauthorized duplication or partial misappropriation.
FIG. 8 is a block diagram showing an example of a conventional encoder/decoder of digital image data comprising an illegal duplication protection mechanism. In an encoder 201, an original image is passed through a Discrete Cosine Transformation (DCT) unit 20, quantizer (Q) 50 and variable word length coder (VWL) 60 to be converted into the compressed digital image data based on MPEG, for example, and is then scrambled in a scrambler 210. The scrambled digital image data can be reproduced only by a descrambler corresponding to the scrambler. The scrambled data is transmitted to a recording/transmission means 70.
The digital data inputted to a decoder 202 is therefore scrambled. In the descrambler 220, the scrambled data is descrambled, and the original compressed digital image data based on MPEG, etc., is re-constructed. The re-constructed data passes through a variable word length decoder 80, reverse quantizer 90 and inverse discrete consine transformation (IDCT) unit 100 so as to give a reproduced image. In this way, in the prior art, illegal duplication was prevented by scrambling data. JP-A-6-121313 (1994) discloses an example where a scrambling technique is applied to an analog picture signal.
Hence, it was previously possible to prevent illegal duplication by scrambling, but due to technical advances, protection of illegal duplication by this method has become ineffective. Specifically, a person who intends to illegally duplicate data analyzes the scrambling algorithm, and is then able to descramble the scrambled data. As a result, a cycle has been created where producers of original data continually develop new scrambling algorithms, and illegal duplicators continually analyze the new algorithms to descramble the scrambled data. Consequently, it has become fundamentally impossible to eradicate illegal duplication. After a scrambling algorithm is analyzed and the data has been descrambled, it may then be duplicated in the usual way. It is also extremely easy to duplicate data which has been duplicated once. Therefore, conventional techniques could not prevent the increasing spread of illegal duplication.